In order to utilize the full bandwidth of the luminance component of composite video signals special circuitry is required to separate it from the chrominance component which shares the high frequency band of the composite signal. Conventional circuitry used to perform this function is in the form of transversal filters, the most common of which is the interline comb filter which obtains the sums and differences of the composite signals delayed, with respect to each other, by one or two integral horizontal line intervals. Interline comb filters perform relatively satisfactorily even when the video signal contains image motion. Interline comb filters, however, produce a particularly objectionable artifact known as "hanging dots". Hanging dots are observed as a line of bright and dark spots reproduced along horizontal edges that undergo a color transition and are caused by incomplete cancellation of chrominance in the luminance signal.
A second type of wideband luminance/chrominance decoder is the frame comb filter which obtains the sums and differences of composite video signals delayed relative to each other by integral frame intervals. Frame comb filters do not exhibit any of the undesirable artifacts produced by interline comb filters if the images represented by the video signal contain no image motion. However, when images do include motion, the frame comb filters generate phantom images and relatively large areas of color dots along moving edges. Numerous methods have been proposed which eliminate motion induced artifacts, all of which require circuitry to detect the occurrence of motion. In general, it is difficult to distinguish between signal noise and image motion, thus, the performance of motion adaptive frame comb filters is only slightly better than that of interline comb filters and at the expense of significantly more circuit hardware.
Finally, a third type of transversal filter for decoding the luminance and chrominance component from composite video takes the form of, e.g. output weighted finite impulse response filters. These filters require a large number of frame delays, for example seven, and though they do not require motion adaptive circuitry, are prohibitively expensive for use in consumer TV receivers.
An object of the present invention is to provide a luminance/chrominance decoding apparatus which exhibits superior performance, does not require motion adaptive circuitry, and is relatively cost effective.